Access the "Quick Settings" menu swiping the screen from top to bottom when the screen is on and unlocked. If you do not see the Wi-Fi icon, swipe down again or click on the gear icon in the lower-right corner (then tap on "Network & internet" and tap Wi-Fi).
Once Wi-Fi is turned on, all available and non-hidden Wi-Fi networks will be displayed below. Select the appropriate Wi-Fi network and following instructions to enter the network password (if available).
Note: some Wi-Fi networks do not require a password.